Several types of digital to analog converters (DACs) that supply an analog output current or voltage that is a linear product of an analog input reference voltage and a digital input word are well known in the art, including the R-string DAC, the R-2R resistance ladder, the current steered DAC and the capacitive based DAC. The R-string DAC comprises a first plurality of resistors in series coupled between two reference voltages supplied by two operational amplifiers. A second plurality of resistors is selectively coupled in parallel to each resistor in the first plurality of resistors. The voltages between each of the resistors of the second plurality of resistors are multiplexed to provide the analog output. However, the parallel combination of unique plurality of resistors creates current errors in the first plurality of series connected resistors. Traditionally, to reduce this current error, the unique plurality of resistors comprise large resistances. This however decreases circuit speed dramatically.
The current steered DAC comprises a plurality of current sources which are switched by digital binary-weighted input signals for providing an analog current to the output. Resistors are typically used to determine the voltage that each current source contributes to the output voltage. However, it is difficult to match the transistors used as the current sources for low current applications.
The R-2R resistance ladder comprises a first plurality of resistors connected in series and a second plurality of resistors, one each coupled between each bit of the binary input signal and a node between one each of the resistors of the first plurality of resistors. Each of the second plurality of resistors have twice the resistance of each of the first plurality of resistors. The analog signal is produced at one end of the first plurality of resistors. However, the total resistance of the ladder is small and highly capacitive, providing a slow, high power circuit.
Thus, what is needed is a DAC having a high current to least significant bit ratio, increased speed without increased power, and wherein the accuracy of the upper bits is independent of the current.